docs: def validate_units(self):
          """Ensure that wavelenth and flux units belong to the
          correct classes.
  
          Raises
          ------
          TypeError
              Wavelength unit is not `~pysynphot.units.WaveUnits` or
              flux unit is not `~pysynphot.units.FluxUnits`.
  
          """
          if (not isinstance(self.waveunits, units.WaveUnits)):
              raise TypeError("%s is not a valid WaveUnit" % self.waveunits)
          if (not isinstance(self.fluxunits, units.FluxUnits)):
              raise TypeError("%s is not a valid FluxUnit" % self.fluxunits)